1. Field of the Invention
The present invention relates generally to lubricant compositions useful in medium speed diesel engines such as commonly found in railroad locomotives, marine towboats and stationary power applications. These engines frequently have silver bearings which necessitate lubricant compositions incorporating specialized silver protective agents to protect against wear, extreme pressure and corrosion of silver parts. However, it is well known that zinc-containing wear agents such as the zinc dihydrocarbyldithiophosphates (typically used in passenger cars) cannot be used for this purpose given their incompatibility with silver bearings. Although chlorine-containing wear agents have been used for silver protection, it is desirable to find alternatives to such chlorinated materials. Thus, the present invention, more particularly, is directed to a lubricating composition, preferably essentially free of zinc dihydrocarbyldithiophosphate compounds, and optionally free of chlorine-containing silver lubricity agents comprising a major proportion of an oil of lubricating viscosity and a minor amount of a silver protective agent comprising an organo-sulfur compound, preferably a thiadiazole compound having the general formula: ##STR1## where x and y (the same or different) are integers from 1 to 5 and R.sub.1 and R.sub.2 (the same or different) are H or C.sub.1 to C.sub.50 hydrocarbyl in combination with the reaction product obtained by reacting a substantially aliphatic, substantially saturated hydrocarbon-substituted dicarboxylic acid wherein the hydrocarbon group contains at least about 20 aliphatic carbons, with (i) an aminoguanidine compound having the formula: ##STR2## wherein R is H or C.sub.1 to C.sub.15 hydrocarbyl, and R' and R" are H or C.sub.1 to C.sub.20 hydrocarbyl (R' and R" being the same or different) or (ii) salts thereof, under reaction conditions giving rise predominantly to formation of a hydrocarbon-substituted 1,2,4-triazole.
The invention is further directed to a method for protecting silver parts in an internal combustion engine by lubricating the same with a lubricant composition comprising a major proportion of an oil of lubricating viscosity and a minor amount of the above-described combination. In accordance with the present invention, the combination results in a synergistic silver lubricity effect and obviates or markedly reduces the need for chlorine-containing silver lubricity agents.
2. Discussion of the Prior Art
Large numbers of medium speed diesel engines in the United States, as well as other countries, utilize silver-plated bearings, especially railway diesel engines. Thus, apart from providing stability against oxidation and protection against the formation of sludge and carbonaceous deposits, crankcase lubricating oils intended for use in medium speed diesel engines must also be formulated with specialized silver protecting agents in order that silver parts in the engine are not attacked either by the additives in the oil or by the dispersed neutralized decomposition products produced during extended engine operation. Such agents, often referred to as silver lubricity agents, protect against extreme pressure, wear and corrosion.
Although it is essential to include a silver lubricity agent in diesel oils intended for use in engines having silver parts, it is well known that such oils must exclude the zinc-containing anti-wear agents mentioned above, such as the zinc dihydrocarbyldithiophosphates, given the known propensity of the latter to damage the silver components of diesel engines. This is explained, for example, in U.S. Pat. No. 4,428,850 (column 1, lines 63-68).
The antagonism between zinc-containing wear inhibitors and the silver parts in diesel engines has been circumvented in the prior art by using alternative silver lubricity compounds, the most common of which are the chlorinated hydrocarbons such as shown in Sung, U.S. Pat. No. 4,171,269. However, while the chlorine compounds of the prior art have been shown to be effective in protecting the silver parts of diesel engines, the Occupational Safety and Health Administration in the United States and other public health agencies throughout the world have expressed concern over potential biological effects of chlorinated compounds. Therefore, an incentive exists to develop novel compositions effective in protecting the silver parts of medium speed diesel engines which overcome the problems or potential problems encountered with the zinc-containing and chlorine-containing wear inhibitors.
A number of patents are thought to be of relevance as background to the compositions and methods described in the present invention but do not address expressly, or even inherently, the problems described above. For example, Knepper et al., U.S. Pat. No. 4,595,523, is directed to a corrosion inhibiting composition comprising a triazole and an amine salt of an acid. While the patent states that "any amine which would form an acid salt which is soluble in an alcohol and inhibits corrosion may be used in the invention," there is no specific mention of carboxylic acid salts of aminoguanidine. Also, the invention of Knepper is clearly directed to a composition which is added to an alcohol fuel. Thus, the patent in no way relates to a lubricant composition, free of zinc compounds, which would be suitable for addition to a diesel engine lubricating composition used in a diesel engine having silver bearings.
Boehringer, et al., U.S. Pat. No. 3,749,702, describes metal deactivators useful as additives in lubricants and, more particularly, describes salts formed from amides prepared from benzoic acid or a substituted benzoic acid with aminoguanidine bicarbonate and an aliphatic or aromatic carboxylic acid. The present invention is not directed to the reaction product of a benzoic acid or its derivatives with aminoguanidine bicarbonate and is clearly distinguishable from Boeheringer, et al.
Lange, et al., U.S. Pat. No. 4,491,527, describes ester-heterocycle compositions useful as "lead paint" inhibitors in lubricants. In particular, at column 4, lines 34-51 (plus the accompanying drawings) the patentee describes the reaction of a substituted carboxylic acid (e.g., polybutylsuccinic acid) with an acyclic heterocycle precursor which cyclizes with the carboxylic acid group to form a heterocyclic compound. An illustrative acyclic heterocycle precursor which may react with an acid or an acid derivative group to form such heterocycles include aminoguanidine and salts thereof, semicarbazide, thiosemicarbazide, carbohydrazide and thiocarbohydrazide, as well as salts thereof such as aminoguanidine bicarbonate. Thus, the composition of Lange, et al. can include the 5-(polyalkenylsuccinic)-3-amino-1,2,4-triazole. The patent, however, is not specifically directed to a lubricant composition for use in diesel engines having silver parts. In fact, at column 12 of the patent, an illustrative lubricant composition of the invention is shown to include a zinc diaklylphosphorodithioate wear inhibitor which would be totally unacceptable in the present invention.
Sung, et al., U.S. Pat. No. 4,256,595, is directed to a diesel crankcase lubricant composition comprising a lubricating oil base and the reaction product of a hydrocarbyl succinic anhydride in which the hydrocarbyl radical has from 12 to 30 carbon atoms, and 5-aminotriazole. The background section of the patent states that it is known to employ a thiadiazole as a corrosion inhibitor for diesel crankcase lubricating oil. The patent, however, does not disclose or suggest a synergistic combination of hydrocarbon-substituted 3-amino-1,2,4-triazole compounds and 1,3,4-thiadiazole compounds, as disclosed in the present invention.
Davis, U.S. Pat. No. 4,136,043, is directed to compositions useful for suppression of copper activity and "lead paint" deposition and lubricants. The compositions are produced by preparing a mixture of an oil soluble dispersant (preferably a substantially neutral or acidic carboxylic dispersant) and a dimercaptothiadiazole, preferably 2,5-dimercapto-1,3,4-thiadiazole. As stated at column 4, lines 24-39, the carboxylic dispersants encompass nitrogen bridged dispersants wherein the nitrogen group is derived from aliphatic, aromatic, heterocyclic and carbocyclic amines as well as substituted ureas, thioureas, hydrozines, guanidines, amidines, amides, thioamides, cyanamides and the like. Davis is not directed to the achievement of silver lubricity in lubricating compositions for diesel engines.
In U.S. Pat. Nos. 3,272,746 and 3,341,542, Le Seur, et al., disclose lubricating oil compositions containing acylated nitrogen compounds prepared, for example, by reacting a substituted succinic acid or derivative thereof with a nitrogen-containing compound, such as ammonia, aliphatic amines, aromatic amines, heterocyclic amines, or carboxylic amines. The resulting detergent composition comprises an oil-soluble, acylated nitrogen composition characterized by the presence within its structure of (A) a substantially hydrocarbon-substituted polar group selected from the class consisting of acyl, acylimidoyl, and acyloxy radicals wherein the substantially hydrocarbon substituent contains at least about 50 aliphatic carbon atoms and (B) a nitrogen-containing group characterized by a nitrogen atom attached directly to said relatively polar group. In Example 38 of these patents, polyisobutene-substituted succinic anhydride, aminoguanidine bicarbonate, and mineral oil were mixed and heated at a temperature of 130.degree. C. (266.degree. F.) to 165.degree. C. (329.degree. F.) for 5 hours. The residue was mixed with mineral oil and heated to 150.degree. C. (302.degree. F.) and filtered. The resulting product was used as a lubricating oil additive and found to be an effective dispersant. These patents each that the mixture of acid-producing compound and the nitrogen-containing reactant is usually heated at a temperature above about 80.degree. C. (176.degree. F.), preferably, within the range of about 100.degree. C. (212.degree. F.) to about 250.degree. C. (482.degree. F.). These patents teach that guanidines are included in sources of nitrogen-containing compounds and present, as examples, guanidine, 1,3-diphenylguanidine, and 1,2,3-tributylguanidine. These patents do not indicate that the resulting product comprises triazoles. Furthermore, these patents do not disclose or suggest synergistic combinations of organo-sulfur compounds and hydrocarbon-substituted 1,2,4-triazoles.